This invention relates generally to digital MTI radar systems and particularly to the digital video cancellation arrangement therein.
It is known in the art that, in MTI radar systems of the type contemplated herein, a reference signal is generated within the receiver of such system, such signal being coherent (that is, in phase) with each pulse in a train of successively transmitted pulses of radio frequency energy. A portion of the received energy resulting from each transmitted pulse is processed with such reference signal to produce a corresponding number of video signals during each range sweep. For convenience, however, the video signals so produced may be considered as a composite video signal made up of different portions, depending on whether the targets are moving or not. The portions of the composite video signal produced by reflections from stationary targets are in constant phase relationship with the reference signal between successive range sweeps. The portions of the composite video signal produced by reflections from moving targets vary in phase relationship with the reference signal between successive sweeps. Therefore, by comparing the composite video signal of a current range sweep with the composite video signal of at least one previous range sweep, cancellation of the portions of the composite video signal resulting from stationary targets may be effected.
The cancelling process referred to above is performed in digital MTI radar systems by a digital canceller. When such a canceller is employed, the composite video signal during each range sweep is digitized by an analog-to-digital (A/D) converter. Therefore, the A/D converter used in present digital MTI radar systems must accommodate the wide dynamic range of the composite video signal.
Various types of A/D converters are described in Skolnik's "Radar Handbook" published by McGraw-Hill, Inc., N.Y., N.Y. (1970) pp. 5-46 to 5-49. As there shown, the type of A/D converter selected for use in any application requires a tradeoff between conversion rate and complexity. Thus, the simultaneous A/D converter operates at the greatest speed but is most complex, requiring 2.sup.N.sup.-1 discrete comparators and associated comparator components for N bit accuracy. Sequential A/D converters, for example, require fewer discrete components than the simultaneous A/D converter; however, the speed of the sequential converter is much lower than the simultaneous A/D converter.